Daniel and Kelly’s Extraordinary Universe - Are there galaxies without dark matter?

Episode Date: December 5, 2023

Daniel and Jorge discuss how dark matter shapes our Universe and whether it would be possible to have a galaxy without it.See omnystudio.com/listener for privacy information.

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and their courageously told stories. Listen to Family Secrets Season 12 on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. Hey, Daniel, most of the stuff in the universe is dark matter, isn't that right? Yeah, it's about 80% of the matter in the universe. And it's all around this, like right here with us, we're immersed in dark matter. Yeah, we are swimming in it. And it's also inside of us, is it? Yeah, it passes through us.

It doesn't bounce off your skin. Does that mean that we're partly made out of dark matter? Ooh, I guess so. That's kind of dark to think about. So if I discover myself, I'm discovering dark matter. Know thyself win a Nobel Prize. Or at least a dark Nobel Prize? Technically, all Nobel Prizes have dark matter inside of them too.

Maybe they'll give you the cash in dark money. Ooh, that got dark quick. Hi, I'm Jorge, I'm a cartoonist and the author of Oliver's Great Big Universe. Hi, I'm Daniel. I'm a particle physicist at CERN and a professor at UC Irvine, and I'll take your money dark or light. Really? You'll take dark money? Isn't that dangerous for a tenured professor? No, that's what ten years for, man. I can take money from dineous individuals. I guess technically the university takes the money and you get a cut of it. I never really had to worry about that.

Like, nobody who got wealthy with dubious techniques has ever offered me a slush fund. So I've never had to really grapple with that question. Wait, how do you know? I mean, you've taken money. Have you done your due diligence? How deep did you go? You know, almost all of my research money comes from the government. And we all know the various crimes that the government has committed.

Exactly. all government money's dark money. Do you believe taxes or theft? Is that where this is going? Well, technically your funding comes from taxpayers, and there must be some sketchy taxpayers out there, so your work is tainted. So you're saying drug dealers who are paying taxes

and indirectly funding my research have made me complicit in their crimes. That's right, yes. Assuming drug dealers pay taxes, which I guess they do if they're laundering money. I don't know. I feel like we need a whole podcast episode just to cover.

dark money. Yeah, and then I suppose everybody's guilty. That's right. We're all guilty of paying taxes, I guess. But anyways, welcome to our podcast. Daniel and Jorge explained the universe, a production of IHeartRadio. In which we try not to overtax your brain while explaining all of the mysteries of the universe. We want to reveal the light universe and the dark universe, the visible and the invisible. We want to show you how the universe is not just what you see around you and experience day to day, but so much more, so much deeper, so many more mysteries waiting to be solved. That's why we explore the universe, the dark corners of it, the light corners, and the sketchy corners of it, because our understanding of the universe is still a little bit sketchy.

Like the mafia corners of the universe? No, like not clearly drawn. Oh, I see. I thought we're going to be talking about like the physics of New Jersey or something. Yeah, there's definitely some dark matters going on out there. How do you eat all those rich cookies and not gain weight, really? It's amazing. Yeah, well, I think the secret is dark chocolate. It's lower in fat, isn't it? Is it, really?

I don't know. You're like, what, why aren't I eating more dark chocolate? I'm not really stopping myself from eating dark chocolate, but that wasn't the reason. I don't see dark chocolate as like diet chocolate. It's chocolate light. Oh, no way, it's chocolate dark. It's dark chocolate. Exactly.

But there are lots of fun questions out there in the universe, not just about how waste management organizations in New Jersey are getting their money and the calories that Daniel is eating. We're wondering about the biggest questions in the universe. Like, where is all the stuff? What is most of the universe made out of? And how is it shaped the night sky that we see today and the galaxy that we live in? That's right, because what the universe is made out of is maybe one of the biggest questions we can ask about the universe. What is this whole place made out of? What are we made out of?

Are we made out of dark matter? Can we exist without dark matter? And we talk on this podcast a lot about dark matter. We have lots of episodes about what it is and where it is and how it works. And one thing we often stress is that dark matter is part of our galaxy, that most of our galaxy is actually dark matter. That has played a big role in the formation of our galaxy. We wouldn't be here without it. Yeah, we do talk a lot about dark matter.

Daniel, what percentage of our episodes would you say we talk about dark matter? is a representative of the amount of dark matter in the universe. Are we ignoring dark matter in a way? Is dark matter underrepresented matter on podcasts? That's a good question. Yeah. Well, especially our podcast, if we're trying to explain the universe.

And the universe is 27% dark matter. You know, we might reach 27% of our podcast being about dark matter. It's been a lot of them. Yeah. I'm pretty sure we don't have two-thirds of them about dark energy. Maybe we should.

Yeah, yeah. That's what everyone is asking for more dark energy episodes. But it is a pretty important part of the universe. It's a pretty important part of our existence because without dark matter, maybe galaxies would not have formed the same way that they form. The Milky Way might not be the same way it is now. But we're always tempted to overgeneral lives

to say that the way we live in our certain situation over here is the way the whole universe works. It's important to take a step back and to ask whether our way of life and our way of galaxies is the only way that it can be. And so today on the podcast we'll be asking the question Are there galaxies without dark matter? Would you call these light galaxies then?

Or galaxies light diet galaxies? I'd call them tragic galaxies because they're probably galaxies where everybody only eats white chocolate. Or milk chocolate. Some chocolate snobs might also call that a tragedy. Also known as Crimes Against Chocolate. Are you saying Hershey's is dark money as well?

Hershey's is garbage man Oh my gosh And there goes our sponsorship deal with Hershey's We were so close to funding this thing For the next 20 years And you had to go and insult them Maybe you could hear my hesitation there I'm being torn between being honest and truthful

On a hard science podcast And pandering to our sponsors And I just had to be honest about it I pass on Hershey's I'd rather have no chocolate than Hershey's That was pretty harsh, though. Garbage. Oh, my gosh. Well, here's the thing. Like, when you eat at, let's say you eat like a chocolate cake at a restaurant. How do you know they didn't use Hershey's chocolate? You can taste it, man. That sour affront to chocolate. It's turned so many people off of chocolate when chocolate is this wonderful, amazing thing. Even milk chocolate can be high quality. It can be amazing. What they make in Pennsylvania, man, it's a crime. Now you're insulting the whole state of Pennsylvania. Let's keep going. See how many people are. I can offend.

Yeah, yeah. Let's move. Let's insult the whole universe. Why don't you? No, fortunately, most of the universe is dark matter and therefore appreciates dark chocolate and so we're good. We have the majority firmly on our side. But doesn't Hershey's own some fancy brands like Sharfenberger or something like that?

I don't know. I hope not. Well, maybe you've been eating Hershey's chocolates all this time. Ah! Plot twist. Anyways, we're talking about dark matter and galaxies. And could there exist galaxies? without dark matter. These would be like galaxies that don't have any dark matter in them or around

them. Yeah, exactly. Just stars and gas and dust and black holes. No dark matter. All right. Well, whether they are tragic or not, we were wondering how many people out there had wondered about this question and if they have any ideas about the answer. Thanks very much to everybody out there who answers these questions, whether or not they are supporters of Hershey's Crimes Against Chocolate. We really appreciate everything you do. If you would like to join this group, just write to me to questions at Danielanhorpe.com. I feel like maybe there's a Hershey's employee out there who listens to our podcast and is now very, very sad.

There's an easy fix for that. Find a new job. Or a new podcast. No, you've got to go to the root of the problem. That's right. Anyway, so think about it for a second. Do you think there can be galaxies without dark matter? Here's what people had to say.

I feel like there probably aren't, only because I know that dark matter, as far as I know, is distributed pretty much evenly throughout the universe. I think it tends to be clustered in galaxies, but I think it tends to be pretty uniform, so I would be surprised if there were galaxies without dark matter. I'm going to say why not, because, like, going back to your episode about uranium on Uranus, there could be a tiny bit here and there in a galaxy, so I'm going to say, I don't know why there wouldn't be dark matter in a galaxy, and I'm also going to say I don't know why there would. I don't believe there are galaxies without dark matter, because dark matter is in most,

my understanding, a general term for unknown matter, which is and makes up the overwhelming majority of the universe. Therefore, I think it is not possible for galaxies not to have at least some dark matter in them. I'm not sure that we've observed any galaxies without dark matter, but I suppose anything could be possible in this crazy universe. It's also possible that all galaxies are without dark matter, and we just don't understand gravity. All right. Most people are skeptical about this question. Yeah, people have the idea that dark matter is everywhere. It's inescapable. You can't get away from it. It seems in our question, although one person has kind of said, why not? That's a good attitude to have. Yeah, that's sort of

the whole attitude about physics. Like, well, maybe everything is different from what we thought or maybe there's something really weird out there that could teach us something new about the universe. I do feel like that is a guiding question in theoretical physics, at least. Why not? Yes. Why not? Sure. Maybe everything is just tiny cats at the quantum scale. That's right. Maybe everything's just made out of her. First, he's chocolate. No. I got to hop somewhere else in the multiverse, if that's the case. All right. Well, let's get going before this podcast gets too dark.

Daniel, give us the basics. What is dark matter for those of us who haven't listened to the 27% of our podcast episodes? Dark matter is fascinating because we simultaneously know a lot about it and very little. Like, we know that there's a lot of dark matter in the universe and we know that it's matter. We know there's something out there that's creating gravity or curvature of space. time, but that it's invisible. It doesn't glow. It doesn't give off light. It doesn't reflect light. We sense it only because we see its gravitational effects on stuff. It's curbing space, which changes how things are moving through that space. We see galaxies rotating much faster than their

gravity would be able to hold them together if there wasn't also dark matter in them, holding them together. On the other hand, we don't know what stuff it is. We know that it has gravity so it's matter, but we don't know if some weird new kind of neutrino or a totally different kind of particle we've never seen before or a thousand new kinds of particles or something that's not even a particle. So we know a lot about it on the sort of cosmological scale, but very, very little or almost nothing about it at the particle level. Right. Dark matter is this kind of mysterious stuff in the universe that we kind of feel its presence. We can see its presence through gravity, but as you said, you can't see it because it doesn't interact with electro-magnetic light or

maybe any of the other forces in the universe and so you can't see it and that's why you call it dark exactly and a bunch of listeners write in with the idea that maybe dark matter is matter in another universe that somehow leaking in to ours remember that dark matter is creating gravity in our universe or changing the curvature of space in our universe which means that it's in that space right it shares that space with us which kind of means that it's in our universe and so we really do know that dark matter is something in our universe that's changing the shape of space we can only see sort of indirectly through gravity which is really frustrating because gravity is the worst way to see things it's so

weak that it makes it very very difficult but wait couldn't it bend our space and not be part of our space that's possible if you overthrow general relativity in our entire understanding of space time general relativity says that matter tells space how to bend and space tells matter how to move and that means matter in our space like in general relativity the curvature of space comes from the energy density in that space. So if you have some like parallel space overlaid on top of it,

which can also bend that space, then it seems to me like it would be part of our space, you know, sort of like that by definition. But yeah, you could augment or throw out general relativity, replace it with something totally different. But the simpler idea is that it's just some kind of mass we can't see. That explains almost everything we see out there in the universe. So it's sort of the best going explanation.

You can always make more Baroque, complicated explanations if you like. Well, it kind of might as well be in another universe, right? Because if it doesn't feel a lot of the same forces we don't, it's just kind of like ghostly matter that's kind of living on top of us. There might be beings made out of dark matter, right? Physicists call the different sectors of the universe. If you have like two different sets of particles that don't interact at all except for through gravity, then we call those like the lights, the visible sector and the dark sector of the universe.

And that's totally possible that you could have a whole complicated physics happening in the dark sector that we can't see. Now, mostly we know that dark matter can't interact with itself. If it did interact with itself, it would form all sorts of complicated structure and do all sorts of interesting things. We think that dark matter is pretty spread out. However, there could be a little component of dark matter. Dark matter may be lots of different kinds of things. And one little component of it might be more complicated and do complicated things like form life or ice cream cones or cats or good chocolate without violating what we've seen dark matter do, which is mostly spread out smooth. All right. Well, the question here today is, can there be galaxies without dark matter? Why is this even a question? Like, are most of the galaxies that we see out there, do they all have dark matter?

Yeah, the galaxies that you see out there in the sky are like tracers. They're basically telling you where the dark matter is in the universe. Remember that dark matter is not something we can see, but it also dominates the universe. Our estimates are that four-fifths of the matter in the universe is dark matter. So if you like spin the wheel and pick a random object in the universe, most likely you're going to get dark matter. It's like overwhelmingly dark matter. So when the universe is forming its structure and the gravity that determines like where things are going to be, it's mostly the gravity of dark matter that decides where things are going to clump together and where things are not going to clump together. Remember, the very early universe is mostly smooth with a few little blobs that are denser than others. The gravity of those over dense pieces pull things together to form structure. And that's where you get like galaxies in one part of space and not galaxies in another part of space.

Where you have galaxies is where you had more dark matter to pull that stuff together to form those galaxies. Yeah, that's pretty wild to think that something we can't see that is invisible to us basically kind of dictates the entire structure of the universe, right? At least at the galaxy level, this dark matter also dictate things like super clusters and beyond. Exactly. And so you can imagine like these invisible wells. like dark matter's curving space, which shepherds the other kind of matter together. So every time you look up at the night sky and you see a galaxy, you should imagine there's an invisible blob of dark matter surrounding that galaxy.

There's a whole halo that's created the conditions to form that galaxy. We've run simulations, for example, of a universe without dark matter, and it doesn't form galaxies after 14 billion years. So quite literally, we would not be alive without dark matter. And now is the same true for things like superclusters and those giant bubbles? of galaxies out there in the universe, is that dictated by dark matter as well? Or is that more of the quantum fluctuations of the universe? Well, both, because there's this cosmic web that tells us where dark matter will be denser and where dark matter will not be dense. You know, these

filaments in some places they overlap and you get these wells where things pooled together. And so the whole cosmic web is dictated by dark matter. And the light matter, the normal matter, barons and quarks and electrons just follows that. And so it's not like a super cluster has a super halo, a supercluster is made of galaxies connected together by these filaments. So it also has filaments of dark matter? Absolutely, yes. There are filaments of dark matter connecting these halos of around each galaxy. Interesting.

All right, well, let's dig into the question of whether a galaxy can be clean of dark matter or whether it's kind of a requirement for a galaxy to form. So let's dig into that. But first, let's take a quick break. I had this, like, overwhelming sensation that I had to call it right then. And I just hit call, said, you know, hey, I'm Jacob Schick. I'm the CEO of One Tribe Foundation, and I just wanted to call on and let her know. There's a lot of people battling some of the very same things you're battling.

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Could you maybe have a scrappy little galaxy out there that was like, no, I don't care about dark matter. I'm just going to gather all these, all this gas and dust and on my own without any help. That's kind of what we're asking today, right? Yeah, exactly. And so you talked about how most of the galaxies that we see out there probably have dark matter, right? We think they have dark matter, right? Because they couldn't be holding together without dark matter. That's right.

And it's even more than just most galaxies have dark matter. It turns out galaxies are sort of like extra rich in dark matter. Like most galaxies have more dark matter than the average dark matter density in the universe. Wait, what do you mean? Like, what are some numbers? So if you average over the whole universe, like what fraction of matter in the universe is dark matter? That's 84%. And that's not by counting.

We don't know how many dark matter particles there are. That's by mass. Like what fraction of the mass of stuff in the universe is dark matter? That's about 84%. But when you look at galaxies and you ask like, what fraction of the mass in a galaxy is dark matter? matter. That's more like 91%. So galaxies have like about half as much normal matter as the average normal matter density in the universe. Galaxies are like concentrated blobs of dark matter. And we get these numbers how, like by measuring how fast the galaxies are rotating and kind of guessing how much dark matter you need to hold it all together? Yeah, not so much guessing, measuring, right? But you're right, it's looking at how the galaxy rotates.

We can measure the feed of those stars in the galaxy as they, whizz around the center by looking at their light and seeing how it's red shifted or blue shifted. If you're looking at a galaxy, some of the stars will be moving away from you and some moving towards you. So they'll be red or blue shifted, their light from the Doppler shift. So you can measure their velocities. So you can look at the velocity of stars as they get further and further away from the center. And in order to hold a star at a certain velocity, a certain radius, you need a gravitational force there. So you can calculate exactly how much gravity is needed to hold a star at a certain radius.

So, have all these stars at different distances from the center telling you exactly how much gravity you need to keep those stars going at that speed. And then you can add up how much you can see, like count all the visible stars, and the rest you suppose is dark matter. I guess that's what you mean by guessing. Yeah, inferring, guessing. There's a whole field of statistical inference that we should just call guessing. Yeah.

Well, I mean, you don't actually know how many stars there are in that galaxy, right? it's so far away. You can't see the individual stars. So you're also sort of inferring how many stars there are there. You're guessing a little bit, aren't you? There's always uncertainty in these measurements, absolutely. And you're right that we cannot resolve individual stars, especially near the center where things get very dense. But we can see the streams, right? We can see streams of stars. We have models for how these galaxies work. But absolutely, there's always uncertainty. But the uncertainty in these calculations is tiny compared to the size of dark matter. So there's no uncertainty that there's a lot of

dark matter in these galaxies because remember the fractions we're talking about here like 90% which means you're looking at a star you're measuring its velocity you figure out how much gravity is needed to hold it there so it doesn't fly out into intergalactic space and when you add up all the stars you get like 10% of the gravity you need so there's a huge missing chunk and you're sure it's not just um you know a lot of asteroids or rocks that don't glow yeah that's a great question could dark matter just be normal matter that we're not seeing right just like dark chunks of matter so people have looked for that directly those are called machos massive compact halo objects and we think we would see those occasionally like they would pass in front of stars if there was a

lot of them if they were really big we would have spotted them so people have looked for that kind of stuff and not seen it plus we know something about how much normal matter there was in the very early universe because it dictates the fraction of elements that were produced like the hydrogen and helium and lithium very sensitive to the density of quarks and electrons in the early universe we talked about that once big bang nucleosynthesis so we have a pretty good handle on like how much normal matter there was around and we can explain where most of that is now and the rest of it's got to be dark matter and so basically every galaxy out there that we've seen we see that it's spinning faster than it should or it's holding together more than it should

so we think it has dark matter and now is that true for every galaxy we've seen out there? Turns out there's a pretty wide variety like when you look at galaxies out there some of them have a lot of dark matter and some of them have a huge amount of dark matter there is a variety. It turns out that smaller galaxies tend to have more dark matter

than really massive galaxies. Not by absolute amounts but just relative to how many stars they have. Yeah exactly. Higher dark matter fractions I should say. And that's because galaxies are better at holding onto their dark matter than they are their normal matter. Galaxies are crazy places. There's winds from all the stars, right? Every star is a fusion furnace and pushing out protons. These stellar winds are pushing gas out of the galaxies. Then there are

supernovas going off all the time, blowing things up and pushing things out. There's radiation, really intense radiation from the center of the galaxy that's pushing gas out. So galaxies are basically exploding and they're pushing a lot of their matter out. And so the smaller a galaxy is, the less it's capable of holding on to its normal matter, the less it's capable of resisting these forces that push gas out of the galaxies. Because the bigger galaxies have more gravity basically, right? Exactly. The bigger galaxies are still doing this, but they have more gravity so they can hold onto their normal matter. So smaller galaxies which have weaker gravity lose more of this normal matter. So you look out there at dwarf galaxies, really tiny ones,

they can be like 99% dark matter. But wouldn't the larger galaxies also be better at holding on to their dark matter? Like wouldn't smaller galaxies lose some dark matter eventually? Like it might evaporate or something? Yeah, that depends on what dark matter does. And in this theory, dark matter does nothing but gravity. And so you can't really lose your dark matter. Like to lose your dark matter, you need some force that's pushing out on it.

But gravity is just attractive. So all these forces like the solar winds and the radiation and the supernova basically have no impact on the dark matter. Dark matter. Dark matter just like brushes it right off. Like supernova could happen right next to you and a dark matter particle would be like, whatever, dude. And so we haven't seen any galaxies without dark matter.

So then why are we asking the question, are there galaxies without dark matter? Is it more of like, is it possible to have a galaxy without dark matter? Or are we asking like, could there be galaxies where we haven't noticed it doesn't have dark matter? Yeah. So great question. We're curious about this for lots of reasons. Like, number one, we have a theory about how the structure of the universe came to be and how it made galaxies. and this nice story we told you about over densities clumping together to form galaxies,

et cetera, et cetera. But we'd like to test that. We'd like to make sure that's correct. We're often surprised when we look out in the universe and see how things actually work. And so what we'd like to do is check our predictions about like the dark matter fractions of galaxies against reality and see, is this really the way things work? Also, this really helps us understand what dark matter is because seeing how dark matter varies across the universe can tell us something about the nature of dark matter and help us test various

alternative theories about what dark matter might or might not be. But it's not exactly true that we've never seen a galaxy without dark matter. People are out there looking for these and they found some pretty weird cases. Interesting. All right. What are some of these cases? So there's a galaxy group kind of nearby on cosmic scales. It's 63 million light years away. It's called NGC 1052. And basically, it's an ecliptical galaxy in the Cetus constellation. We've known about it for like 250 years or so, but there's actually a little group of galaxies. It's like a major galaxy with a bunch of little galaxies nearby. They call these dwarf galaxies. So it's a whole group. So they call it a group of galaxies. And these little galaxies are actually ultra-diffuse galaxies. That means they're galaxies

that are not very bright. They have very few stars in them. And these ultra-diffused galaxies near this NGC group, they think might have no dark matter in them. at all. Why do they think that? So they look at the rotations of these galaxies and they do that calculation and they estimate zero dark matter. Like every time you're doing this, you're not assuming the dark matter, you're measuring it. And sometimes it comes out 90%, sometimes 84%, sometimes 99%.

In this case it comes out close to zero or consistent with zero. So they think these are little galaxies that have no dark matter in them at all. So that's pretty wild. That means that you can't have a galaxy without dark matter. Yeah, it's fascinating because remember our theory of, of galaxy formation is that basically every big galaxy is a merger of a bunch of small galaxies. Big galaxies don't like form all at once in a single collapse. You have a bunch of baby galaxies which then merge to make bigger and bigger galaxies. So like a bottoms up approach.

And so if your big galaxy ends up with a lot of dark matter in it, that means that the little galaxies that made it should each have their own dark matter. And we look at a dwarf galaxies and we mostly see them having dark matter. In fact, some of them have a lot. So it is really weird to see these little galaxies without any dark matter. at all. And the question is like, did they form this way or did something happen to strip them of their dark matter? Or maybe they formed later in the universe? Yeah, exactly. And so that's a fascinating question. And so there's a group that's done a study of these. And they have a theory about how these little diffuse galaxies ended up without any dark matter in them. What's the theory? So the theory

is basically a mini version of the bullet cluster. If you remember, the bullet cluster is this famous example that really convinced a lot of people that dark matter was a real thing. It was a cluster of galaxies, that collided with another cluster of galaxies and we saw that what happened to the gas and the dust and the dark matter was very different. So the gas and the dust interacted and created collisions. The dark matter passed right through because it doesn't interact at that level. Gravity's not strong enough. So basically separated the dark matter from the normal matter. So the bullet cluster, now you have a blob in the middle with a bunch of normal matter in it and then you have dark matter on both sides. So we can see through gravitational lensing. So they think that might be similar to what happened in this case.

maybe there was a big collision between two other objects and these two things that we're seeing now they call them df2 and df4 are basically the results of that like chunks of stars and gas and dust that got stripped of their dark matter in a collision and then tossed aside well why wouldn't some little bit of dark matter go with them because the dark matter and the normal matter have very different experiences in a collision like dark matter basically passes the right through there's not really a collision when it comes to dark matter it's like two ghosts just phasing through each other Whereas two people bumping into a hallway are going to change their direction. So imagine you have like a ghost inside you and somebody else is a ghost inside them and

you have a collision in the hallway, the ghosts just keep on going and the living people bounce off each other. Now you know, you're separated from your ghost. But there's so many that they discovered, did they all get that way from the collision? So they've only found these two and they have this reconstruction of the collision that suggests that these two things happen somewhere near NGC and created this collision. and it should have also created a bunch of other ultra-diffuse galaxies that they should be able to spot

that there should be like five or six of these that came out of the collision that also have no dark matter. So they're going to go and look for those. So you said these are dwarf galaxies? Yeah, they're ultra-diffuse galaxies. They're also dwarf galaxies. They're small and they're not very bright.

So in a way they kind of got made later or not? Are they as old as the universe? Well, they think this collision happened about 8 billion years ago. And so how you age these things, I guess depends. Like the way they are now started about 8 billion years ago. They, of course, have some progenitor, something that they came from, right? There was a larger object they were a part of, which definitely had dark matter in it. And their dark matter is now sprayed in some other direction.

So they become separated from their dark matter. And this must have been a pretty mammoth event. I mean, they reconstruct this thing, and it's like a collision at 300 kilometers per second of these huge cosmic objects. So these are galaxies that had dark matter, but then they got stripped away of their dark matter. And so that shows that, hey, you can have a galaxy without dark matter. But I guess maybe the larger question is, can you form a galaxy without dark matter? Right. And so as you say, that's the deep question about the nature of the formation of structure in the universe.

And so far, the answer to that is no. We do not think it's possible to form a galaxy without dark matter. We think you need that dark matter around to gather enough gas and dust to make stars and to make a galaxy. that without dark matter, normal matter doesn't have enough gravity to have formed galaxies this early in the universe. If you had a universe without dark matter or a big section of it without dark matter and just normal matter, it would form galaxies eventually,

but it would take a lot longer to do so. But as I understand it, during the Big Bang, things were really hot and dense, and there were pockets of things, and there were quantum fluctuations, which maybe created pockets of extra densities here and there. Couldn't there have been a pocket of extra density of normal stuff, but not dark matter, that then when the universe blew up, it became a galaxy

without dark matter. Like during the Big Bang, why does the normal matter have to follow the dark matter? I guess it does in general because dark matter dominates because it's just so much more of it. So it basically like sets the scene for everything. But are you right, it is theoretically possible to have a downward fluctuation in the dark matter and an upward fluctuation in the normal matter. So you get some region of space where you have like extra super dense normal matter and almost no dark matter, that's possible, yeah. And so in principle, that could happen. And if it had enough matter, then it would form a galaxy on its own.

So in principle, that's not impossible. But we've never seen that. And I don't know what the chances are of that happening theoretically. Like how many galaxies have we done this calculation to make sure that it has dark matter in it? Yeah, that's a great question. We've measured the rotation velocity of thousands and thousands of galaxies. But that's a tiny fraction of the number of galaxies that are out there and the number of galaxies we can see. Most of the galaxies we can see we can't measure their rotation velocity

because you're looking at like one pixel or two pixels. You need to be able to sort of resolve the whole galaxy so you can see light from one side versus light from the other side. So it's tricky. But yeah, we haven't looked at that many galaxies as possible. There are galaxies out there that did really form without dark matter. All right. Well, it seems like we kind of answered the question of the episode,

which is can you have a galaxy without dark matter? The answer is yes. You can have maybe galaxies that had dark matter, then they lose it or they get left behind by the dark matter and so they're dark matter less. Or maybe they could have formed at the beginning of the universe in theory, but we haven't seen one yet. All right. Well, let's dig into what this all means about our understanding of dark matter and also gravity and whether or not it needs to be overhauled.

But first, let's take another quick break. I had this like overwhelming sensation that I had to call it. call her right then. And I just hit call, said, you know, hey, I'm Jacob Schick. I'm the CEO of One Tribe Foundation, and I just wanted to call on and let her know there's a lot of people battling some of the very same things you're battling. And there is help out there. The Good Stuff podcast, Season 2, takes a deep look into One Tribe Foundation, a non-profit fighting suicide in the veteran community. September is National Suicide Prevention Month, so join host Jacob and Ashley Schick as they bring you to the front lines of One Tribe's mission.

I was married to a combat army veteran, and he actually took his own life to suicide. One tribe saved my life twice. There's a lot of love that flows through this place, and it's sincere. Now it's a personal mission. I don't have to go to any more funerals, you know. I got blown up on a React mission. I ended up having amputation below the knee of my right leg and a traumatic brain injury because I landed on my head. Welcome to Season 2 of the Good Stuff.

Listen to the Good Stuff podcast on the IHeart Radio app, Apple Podcast, or wherever you get your podcast. Your entire identity has been fabricated. Your beloved brother goes missing without a trace. You discover the depths of your mother's illness, the way it has echoed and reverberated throughout your life, impacting your very legacy. Hi, I'm Danny Shapiro. And these are just a few of the profound and powerful stories

I'll be mining on our 12th season of Family Secrets. With over 37 million downloads, we continue to be moved and inspired. by our guests and their courageously told stories. I can't wait to share 10 powerful new episodes with you, stories of tangled up identities, concealed truths, and the way in which family secrets almost always need to be told. I hope you'll join me and my extraordinary guests

for this new season of Family Secrets. Listen to Family Secrets Season 12 on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. a foot washed up a shoe with some bones in it. They had no idea who it was. Most everything was burned up pretty good from the fire that not a whole lot was salvageable. These are the coldest of cold cases,

but everything is about to change. Every case that is a cold case that has DNA. Right now in a backlog will be identified in our lifetime. A small lab in Texas is cracking the code on DNA. Using new scientific tools, they're finding clues in evidence so tiny, you might just miss it. He never thought he was going to get caught,

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Hello, it's Honey German, and my podcast, Grazacus Come Again, is back. This season, we're going even deeper into the world of music and entertainment with raw and honest conversations with some of your favorite Latin artists and celebrities. You didn't have to audition? No, I didn't audition.

I haven't audition in, like, over 25 years. Oh, wow. That's a real G-talk right there. Oh, yeah. We've got some of the biggest actors, musicians, content creators, and culture shifters sharing their real stories of failure and success. You were destined to be a start.

We talk all about what's viral and trending with a little bit of chisement, a lot of laughs, and those amazing vibras you've come to expect. And of course, we'll explore deeper topics dealing with identity, struggles, and all the issues affecting our Latin community. You feel like you get a little whitewash because you have to do the code switching?

I won't say whitewash because at the end of the day, you know, I'm me. But the whole pretending and code, you know, it takes a toll on you. Listen to the new season of Grasasas has come again as part of my Cultura Podcast Network on the IHartRadio app, Apple Podcasts, or wherever you get your podcast. All right, we're asking the question,

can there be a galaxy with no dark chocolate? And the answer is probably, but that's not a universe Daniel wants to live in. No, that's right. Transport me somewhere else in the multiverse, ASAP. What if you end up in a universe where there's only milk or white chocolate? Just keep smashing that button until I get somewhere good. No, no, you only get one trip.

Would you take the risk? I'm pretty happy with our universe, you know. It's a pretty good one. We're talking about whether galaxies can exist without dark matter, and the answer is yes, they can be stripped away of their dark matter, or theoretically they could form in the early universe, but we haven't seen one yet. And so maybe probably not.

What do you think happened? Why haven't we seen any? If they can form without dark matter, Why haven't we seen any? I think that'd be really unlikely. I mean, the kind of fluctuations we're talking about are very, very large. In the early universe, you had just sort of like energy and then it decays into matter as the universe expands and cools.

And every kind of matter is sort of made uniformly. So you get more dark matter made and less normal matter. But in order to have no dark matter made, you'd need a really big fluctuation. You expect to get like 85% and you get zero. It's like flipping a coin a hundred times in a row and getting only heads instead of half heads. and half tails. It's pretty unlikely. Now, the universe is really big, of course. So that means that eventually it's going to happen, especially if the universe is infinite.

But it's so unlikely that it's not going to be the first kind of galaxy we see out there or even in the first tranche. Eventually, we might spot one. Well, as you said, it is possible to have a galaxy without dark matter. We've seen it in some ultra-diffused galaxies. And they've done the measurement on these galaxies, right? They've measured how fast it's spinning and they're pretty sure there's no dark matter in them. Yeah.

Now, there were a series of papers where people said, oh, there's no dark matter. And another group did a different measurement said, no, there is some dark matter. And then there were follow-up papers arguing. And now they're pretty sure there's no dark matter in these. But there's always somebody out there who disagrees. I mean, it's astronomy after all. That's right. They're all just guessing.

They're all just doing their best statistical inference. That's right. That's a great word for guessing. No, I'm just kidding. Best guessing. How about that? Best guessing. Nobody has a better guess.

You know those whole departments of people who do nothing but statistics for a living? I'm trashing Hershey's, but you're trashing statistics, man. No, there's nothing wrong with guessing. There's nothing wrong with you. How do you know the universe is the way it is? We're just guessing. A best guess doesn't mean that you're making things up randomly. You're just using the best information you have to make a best estimate or inference, right?

I suppose so. I think if it's very well informed, it's not really a guess, you know? But you're not 100% sure. It's also not a fact. Yeah, that's true. That's why we use statistics to describe our uncertainties. Anyway, one of the things we are uncertain about is the nature of dark matter. Like a lot of the stuff we talk about for dark matter is kind of unsatisfyingly indirect.

And a lot of people out there treat dark matter like it's some placeholder, not a real theory of the universe. Because we never see it directly. We can't really grapple with it and grasp it directly. Right, right. Well, I think it's interesting that we have, or that astronomers have found galaxies without dark matter because it almost gives you kind of like a test case to confirm that the other galaxies that we have seen with dark matter actually have dark matter. And it's not just some weird, you know, fluke or mistake in our theory of gravity. Yeah, it's a cool test case.

It's like a control, right? Would you see galaxies without dark matter if they were there? So it's nice to have some verification that we're seeing that. It's also, as you say, a great testbed for comparing various theories of dark matter, which make different predictions about what would happen in these scenarios. Yeah, different guesses about dark matter. Different ideas, different theoretical concepts. All right, well, how do these galaxies help us decide what dark matter is made out of?

Well, one of the most popular alternatives to dark matter as a theory of matter, some kind of stuff in the universe, is an alternative theory of gravity to say, well, there's no other stuff in the universe. We're seeing everything there is. It's just that gravity works differently from what we expected. Because remember, the argument for dark matter is like, we understand gravity, and there's a lot more gravity than we can explain with the visible stuff. So there must be more stuff.

There must be invisible stuff creating that gravity. But what if instead we just don't understand how gravity works, and it can be explained by all the visible stuff if you tweak your theory of gravity. Meaning, like, what if gravity just gets stronger, the bigger the distances, that might account for why galaxies are holding on together without needing dark matter. That's kind of the idea, right? That's kind of the idea. More specifically, there's this theory called Mond modified Newtonian dynamics that suggests

that gravity is mostly like Newton described, but there are some tweaks. It depends on the acceleration of these objects. And for some accelerations, gravity gets stronger or weaker. And, you know, it's a little baroquely like Adides' turn and these tweaks, basically to explain these rotation. curves. Say like, oh, these stars are accelerating more than those stars. So if we change the way gravity works, can we describe the rotation curves that we see? The answer is yes. You can devise a theory to describe the rotation curves that explain how these galaxies are rotating without needing dark matter if you tweak gravity, right? You have to tweak something. Either change the amount

of matter that's there or you change the way gravity works. Well, first of all, you just insulted the whole period of human history, the Baroque period. I meant that in a positive way. Right. But second of why, I know that we've talked about before how, you know, maybe Monde modified Newtonian dynamics could replace dark matter, but we confirm dark matter in other ways, right? Exactly. So Monde is a success in describing the rotations of galaxies by making these beautiful, baroque extensions to Newton's theory. But there's lots of other ways we've seen dark matter, like in the ripples of the cosmic microwave background from the very early universe. we can see how that early universe plasma was sloshing around, and that depends very sensitively on the amount of dark matter, which sloshes differently in that plasma than normal matter did.

And Mond cannot explain that. Also, the bullet cluster shows us that dark matter can be separated from normal matter. It's not just a different way that gravity works for normal matter. It really is something else with its own gravity. So Mond really struggles to explain everything that the theory of dark matter can explain, But it's still a popular alternative. And this is another way. These dark matter-free galaxies are another way to draw contrast between what Mondt predicts and what

dark matter predict. Because according to Mond, there is no dark matter and gravity only depends on the visible matter. And so these ultra-diffuse galaxies with no dark matter should behave the same way all the other galaxies do because there's no dark matter in any of them. But we do see a difference. We see that these guys are rotating more slowly, right? And so Mon struggles to explain how slowly rotating these galaxies are. without any dark matter.

Whereas dark matter can explain all of it. He's like, well, this has more dark matter that has less dark matter. So because dark matter can be variable in the universe, some galaxies have more and some have less, whereas the rules of gravity have to be the same. Mon is sort of hamstrung and can't really explain the variation of all these galaxies. Well, I feel like Mon was already kind of dead in the water for all these other reasons for a while. But it is kind of interesting that seeing a galaxy without dark matter almost kind of helps

proved that it exists. Yeah, that is really interesting. And I agree with you that Mon is not a theory we should take terribly seriously and Dark Matter is overwhelmingly the better guess

for what's going on in the universe. Yeah, there you go. See, I brought you on board. I'm loving that word now. You're like, I guess. I guess so. But, you know, full caveats, there are some things

that Dark Matter can't explain. There are a few galaxies out there that don't make any sense that no dark matter can really explain. Some people think that some hybrid, like mostly Dark Matter with a little bit of Mond is what we need to explain everything that's out there in the universe.

And so it's best to keep an open mind. It's also always nice to find a new way to test our understanding of dark matter and gravity in general. And so these galaxies without dark matter are a nice test bed for that. Interesting. You can call the new theory Darkmond. All right. Well, another interesting example of how the universe just always has surprises. Like you think that maybe you need dark matter to have a galaxy,

but suddenly one day you find galaxies without dark matter. And it makes you think, and it actually maybe helps you confirm the existence of something as mysterious as dark matter. And it goes to show you that the universe does all these experiments for us. We can just look up in the night sky and find examples of galaxies smashing into other galaxies or black holes colliding. All these things are wonderful experiments that help reveal the nature of the universe, the rules that it follows, and how it all works. Yeah, I guess. Or you guess. In the end, aren't we all just guessing, man?

All right. Well, we hope you enjoyed that. Thanks for joining us. See you next time. For more science and curiosity, come find us on social media where we answer questions and post videos. We're on Twitter, Discord, Insta, and now TikTok. And remember that Daniel and Jorge Explain the Universe is a production of IHeart Radio. More podcast from IHeartRour.

visit the iHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows. Every case that is a cold case that has DNA. Right now in a backlog will be identified in our lifetime. On the new podcast, America's Crime Lab, every case has a story to tell. And the DNA holds the truth. He never thought he was going to get caught. And I just looked at my computer screen. I was just like, got you.

Jim. This technology is already solving so many cases. Listen to America's Crime Lab on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. Tune in to All the Smoke Podcasts, where Matt and Stacks sit down with former first lady, Michelle Obama. Folks find it hard to hate up close. And when you get to know people and you're sitting in their kitchen tables and they're talking like we're talking. You know, you hear our story, how we grew up, how I grew up. And you can, you get to know, you. get a chance for people to unpack and get beyond race. All the Smoke featuring Michelle Obama. To hear this podcast and more, open your free IHeartRadio app.

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